Patent Application
20230039108
This invention relates to the field of perpendicular magnetoresistive elements. More specifically, the invention comprises perpendicular magnetic tunnel junctions (pMTJs) having a novel recording structure having a soft-magnetic adjacent layer to enhance coherent domain reversal of the magnetic free layer for spin-transfer-torque (STT) or spin-orbit-torque (SOT) magnetic-random-access memory (MRAM) using perpendicular magnetoresistive elements as basic memory cells which potentially replace the conventional semiconductor memory used in electronic chips, especially mobile chips for power saving and non-volatility.
In recent years, spin-transfer torque magnetic random access memories (hereinafter referred to as STT MRAMs) using the magnetoresistive effect of ferromagnetic tunnel junctions (also called MTJs) have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can cope with high-speed reading and writing, large capacities, and low-power-consumption operations. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a free layer having a changeable magnetization direction, an insulating spacing layer, and a fixed reference layer that is located on the opposite side from the free layer and maintains a predetermined magnetization direction. The insulating spacing layer serves as a tunneling barrier (TB) in a ferromagnetic tunnel junction. In a SOT MRAM, there is an additional SOT layer immediately located on a surface of the free layer, which is opposite to a surface of the free layer where the insulating spacing layer is provided. SOT can be a thin layer made of heavy transition metal layer such as W or Ta, Pt, etc., or a layer of topological insulating material such as BiSB.
As a write method to be used in such magnetoresistive elements of a STT MRAM, there has been suggested a write method (spin-transfer torque switching technique) using spin momentum transfers. According to this method, the magnetization direction of a free layer is reversed by applying a spin-polarized current along a certain direction to the magnetoresistive element. Furthermore, as the volume of the magnetic layer forming the free layer is smaller, the injected spin-polarized current to write or switch can be also smaller. Accordingly, this method is expected to be a write method that can achieve both device miniaturization and lower currents. In a SOT MRAM, an electric current flows in the SOT layer, which is a paramagnetic layer, to generate a spin-polarized current and inject it into its adjacent free layer, which is a ferromagnetic layer. The spin-polarized current then exerts a torque on the magnetic moment to reverse it. Theoretically, a coherent domain switching or reversal is better than non-uniform domain switching in terms of write power and speed.
Further, as in a so-called perpendicular magnetic tunnel junction (pMTJ) element, both two magnetization films, i.e., the free layer and the reference layer, have easy axis of magnetization in a direction perpendicular to the film plane due to their strong magnetic crystalline anisotropy (shape anisotropies are not used), and accordingly, the device shape can be made smaller than that of an in-plane magnetization type. Also, variance in the easy axis of magnetization can be made smaller. Accordingly, by using a material having a large perpendicular magnetic crystalline anisotropy, both miniaturization and lower currents can be expected to be achieved while a thermal disturbance resistance is maintained.
Perpendicular spin-transfer torque magnetic random-access memory (pSTT-MRAM) based upon above pMTJ elements is an attractive alternative to existing memory-storage technologies because of its high performance, nonvolatility and scalability. MgO-based magnetic tunnel junctions (MTJs) having a highly textured sandwich structure: BCC (100) CoFe (reference layer)/rocksalt (100) MgO (tunnel barrier layer)/BCC (100) CoFe (free layer), exhibit high magnetoresistance (MR) ratios due to the so-called coherent tunneling effect and are considered to be essential for pSTT-MRAMs. MTJs having perpendicular magnetic anisotropy (PMA) enable low switching-current densities required for magnetization reversal and high thermal stabilities of the storage cells. A magnetic stack with an out-of-plane magnetization is energetically unfavorable because of its strong demagnetizing fields, and therefore strong PMA is essential, in the form of either intrinsic magnetocrystalline anisotropy or interface effects.
There has been a known technique for achieving a high MR ratio in a pMTJ element by forming an underneath MgO tunnel barrier layer and a BCC (body centered cubic) or HCP (hexagonal close-packed crystal)-phase cap layer that sandwich a thin free layer (FL) having an amorphous CoFeB ferromagnetic film and accelerate crystallization of the amorphous ferromagnetic film to match interfacial grain structure to MgO layer through a thermal annealing process. The film of the MgO tunnel barrier layer is polycrystalline, with each grain oriented such that the (100) crystallographic direction is parallel to the film normal, which is also called (100) texture. The FL crystallization starts from the tunnel barrier layer side to the cap layer and forms a BCC (body centered cubic) CoFe grain structure having a perpendicular magnetic anisotropy, as Boron elements migrate into the cap layer. Similarly, a thin reference layer (RL), that is located on the opposite side from the free layer, is sandwiched between the MgO tunnel barrier layer and an underneath Boron-absorbing layer having a BCC or HCP phase, and during the same thermal annealing process, the RL crystallization starts from the tunnel barrier layer side to the Boron-absorbing layer and forms a BCC CoFe grain structure having a perpendicular magnetic anisotropy, as Boron elements migrate into the Boron-absorbing layer. Accordingly, a coherent perpendicular magnetic tunnel junction structure having a (100) textured structure of CoFe BCC (100)/MgO rocksalt (100)/CoFe BCC (100) is formed. By using this technique, a high MR ratio can be achieved. However, boron atoms, as impurities, may partially stay at boundaries between CoFe grains which decreases exchange constants in these magnetic materials, and may also partially stay at boundaries between MgO grains which decreases coherent tunneling effect. Here, and thereafter throughout this application, each element written in the left side of “I” is stacked below an element written in the right side thereof.
In order to maintain a predetermined magnetization direction for the RL layer, a high perpendicular magnetic anisotropy (PMA) material must be provided underneath the Boron-absorbing layer that is typically no more than 0.3 nm and magnetically coupled to the RL layer to pin the magnetization of the RL in a predetermined perpendicular direction. A layer made of a high perpendicular magnetic anisotropy (PMA) material is also called as a pinning layer. A conventional method of forming such a pinning layer is briefly summarized as follows. First, a buffer layer is provided on top of a bottom electrode and comprises a property that inhibits a propagation of a crystalline structure of the bottom electrode into the high perpendicular magnetic anisotropy (PMA) material, and a seed layer is provided on top of the buffer layer and promotes a desired crystalline orientation growth for the high perpendicular magnetic anisotropy (MIA) material. The crystalline orientation along the growth axis (e.g., the texture) should be uniform between the seed layer and the high perpendicular magnetic anisotropy (PMA) material. The texture may be measured in a material by using x-ray diffraction, and the terms “high” or “good” texture may be used to describe uniform crystalline orientation along the growth axis that has a certain crystalline symmetry perpendicular to the plane. Typically, a magnetic material may exhibit a special crystalline axis in which the magnetization prefers to be directed, referred to herein as an “easy axis.” Therefore, if a strong PMA is desired, the direction of crystal growth, or the growth axis, extends in the same direction as the direction in which the easy axis points. The direction of the crystal growth is important not only for providing a strong PMA in the magnetic crystalline material, but also for providing a strong PMA at an interface of a crystalline material.
Materials in the first category, displaying intrinsic magneto-crystalline anisotropy, include L10-based materials such as FePt L10 superlattice, and those in the second category, containing interface effects, include (Co/X)n multilayer structures, where X represents Pt, Pd or Ni metals, and n is the number of repetitions. More specifically, in the case of (Co/Pt)n, its FCC (face centered cubic) (111) crystal orientation direction achieves the highest PMA from the crystalline point of view and also from the interface anisotropy point of view, while in the case of FePt L10 superlattice, its (100) crystal orientation direction (texture direction) achieves the highest PMA for the FePt material. (Co/X)n multilayer structures have two distinct advantages over L10 materials: (1) their close-packed growth plane makes them easier to fabricate, and (2) their simpler stack design, which accommodates a choice of thicknesses of the constituent layers, of n, and of X, allows much greater versatility. And more, since forming of FePt L10 superlattice requires a thermal annealing at a very high temperature (typically above 600 degrees Celsius) as in a conventional method which would cause damage to the underneath integrated CMOS circuitry, a popular choice of high perpendicular magnetic anisotropy (PMA) material is (Co/X)n. However, (Co/X)n multilayer structures have two significant shortcomings that undermine their application to pSTT-MRAM devices. First, their PMA properties deteriorate or even disappear when the multilayer structures are subjected to temperatures as low as 300° C. Second, because their close-packed growth plane has an FCC (111) texture, it is difficult to form a BCC (100) textured CoFe reference layer directly upon it. It is noted that a BCC (100) textured CoFe is essential for a high MR ratio.
Due to the fact that a desired reference layer (RL) texture is BCC (100) of 4-fold symmetry and a desired high perpendicular magnetic anisotropy (PMA) material (Co/X)n, texture is FCC (111) of 3-fold symmetry, a texture breaking layer must be provided between the RL and the PMA material, that inhibits a propagation of the FCC (111) texture of (Co/X)n into the RL. Fortunately, many Boron-absorbing materials, such as W, Mo and Ta, are also good texture breaking materials having BCC crystalline structures, and therefore these materials are widely used as a thin texture-breaking Boron absorbing (TBBA) layer inserted between a reference layer made of amorphous CoFeB and a pinning layer made of FCC (111) PMA material. The TBBA layer is thin enough so that the magnetization of the reference layer is strongly parallel-coupled to the magnetization of the pinning layer in order to yield high temperature thermal stability (usually up to about 400 degrees Celsius).
A conventional pMTJ element comprises a magnetic pinning structure (PS), a tunnel barrier (TB), and a magnetic free layer (FL), wherein the PS is formed by sequentially depositing a perpendicular synthetic anti-ferromagnetic (pSAF) stack, a TBBA layer and a CoFeB reference layer. Further, a commonly used pSAF stack has the structure: seed-layer/(Co/X)m/Co/Y/Co/(X/Co)n, where X represents Pt, Pd or Ni metals, m and n are integers (normally m>n), and Y represents a Ru (or Rh, Ir) metal spacer that provides a perpendicular RKKY coupling between the two magnetic multilayer structures (Co/X)m/Co and Co/(X/Co)n. As a prior art, the conventional full film pMTJ element (100) is illustrated in
The purpose of inserting a traditional texture breaking layer having BCC crystalline structure, such as Mo, W and Fe, between the PMA material and the reference layer is to allow the crystalline transition between the FCC (111) textured PMA of 3-fold symmetry and the BCC CoFe next to the MgO barrier. However, the material of the texture breaking layer preferably forms BCC (110) texture instead of the desired BCC (100) texture which has 4-fold symmetry, due to both of the lattice-matching epitaxial growth at FCC (111)/BCC (110) metal interfaces and the BCC (110) being its close-packed plane. In fact, the distances between the neighboring atoms in the Mo BCC (110) unit cell are 3.147 Å/2.72 Å/2.72 Å, respectively; the distances between the neighboring atoms in the W BCC (110) unit cell are 3.16 Å/2.74 Å/2.74 Å, respectively; and the distances between the neighboring atoms in the Fe BCC (110) unit cell are 2.867 Å/2.48 Å/2.48 Å, respectively, all of which have small lattice misfits with the lattice constant 2.69 Å of the [Co/Pt]n surface unit cell (primitive FCC (111) unit cell). The BCC (110) texture in these texture breaking layers would further result in an undesirable, close-packed BCC (110) texture in the CoFe reference layer upon it. To mitigate this problem, an amorphous CoFeB has to be used to replace CoFe as a reference layer material, and the rocksalt (100) textured MgO tunnel barrier layer helps CoFeB crystallization and forms BCC (100) CoFe grain structures in a thermal annealing process. However, due to the undesired BCC (110) textured breaking layer, the reference layer has a mixed texture of BCC (100) and (110) grains after a thermal annealing process. For this reason, an amorphous texture breaking layer, such as FeTa, FeTi and FeZr (see U.S. Pat. No. 9,755,140), has been proposed to suppress crystallization at the interface between the texture breaking layer and the CoFeB reference layer, while only the rocksalt (100) textured MgO tunnel barrier layer helps CoFeB crystallization during annealing. However, the amorphous material of the texture breaking layer may change into nano-crystalline during a thermal annealing process, which would also lead to a certain amount of BCC (110) texture grains in the CoFeB reference layer. And more seriously, Ta, Ti and Zr atoms easily diffuse into MgO via Mg substitutional sites even at room temperature, which would cause a degradation of MR ratio.
There is a different technique proposed by S. Parkin (see U.S. Pat. No. 7,357,995) to form a coherent magnetic tunnel junction structure having a texture structure of CoFe BCC (100)/MgO rocksalt (100)/CoFe BCC (100). In this method, before deposition of the first CoFe layer, a special seed layer, IrMn layer, grows highly oriented in the (100) orientation when deposited on a buffer layer of Ta or TaN/Ta that is first deposited on a substrate of an amorphous material such as SiO2. Using this combination of under-layers, the CoFe layer is textured in the (100) orientation and the MTJ displays a high MR ratio. However, the total thickness of the combined under-layers, or similar type of seed layer structure, is too large to make the reference layer magnetization strongly coupled to the pinning layer magnetization, and moreover, the IrMn layer, as an anti-ferromagnetic material (AFM), would pin the magnetization of the reference layer in an in-plane direction, which disqualifies this technique for perpendicular pMTJ elements.
It is very challenging but desirable to develop a (100)-textured (i.e., cube-textured) layer to replace the traditional BCC (110)-textured TBBA layer in order to achieve a highly BCC (100) textured RL which is strongly pinned by the pSAF through magnetic exchange coupling. This (100)-textured layer must have a self-assembling capability to form a (100) texture through a non-epitaxial texturing method. It is also very challenging but desirable to develop a novel recording structure to replace the traditional CoFeB free layer in order to achieve a highly (100) textured magnetic free layer having a high exchange constant and an additional assisting structure, which are essential for coherent domain switching of the magnetic free layer when a spin-polarized write current is applied.
An objective of the disclosed invention is to solve the above-disclosed challenges faced by pMTJ elements, especially magnetic pinning structures and magnetic recording structures. Further and/or alternative objectives may be understood from the following.
One preferred embodiment is a pMTJ element comprising a magnetically fixed pinning structure that consists of a perpendicular synthetic anti-ferromagnetic (pSAF) stack, an oxide buffer (OB) layer and a cube-textured reference layer, wherein the pSAF stack includes a FCC (111) PMA material multilayer such as seed-layer/(Co/X)m/Co/Y/Co/(X/Co)n, where X represents Pt, Pd or Ni metals, m and n are integers (normally m>n), and Y represents a Ru (or Rh, Ir) metal spacer, and the cube-textured reference layer includes a texture starting (TS) layer and a spin polarization (SP) layer. Both of the TS layer and the SP layer are made of materials selected from the group consisting of ferromagnetic materials and ferrimagnetic materials. The TS layer comprises either a highly BCC (100) textured material or a L10 superlattice (100) textured material. The SP layer comprises a highly BCC (100) textured material. The OB layer sandwiched between the pSAF stack and the cube-textured reference layer is thin enough to produce a strong magnetic parallel-coupling between the reference layer and the pSAF stack, so that the cube-textured reference layer is pinned in a predetermined perpendicular direction. Preferably, the OB layer is a magnetic oxide, such as iron oxide or cobalt oxide, which produces a strong magnetic parallel-coupling between the reference layer and the pSAF stack. The TS layer comprises at least one of an iron (Fe) layer, a cobalt (Co) layer, an alloy layer of iron cobalt (FeCo), a laminated layer of (Fe/Co)n, an alloy layer of FeX, a laminated layer of (Fe/X)n, where X represents Pt, Ni or Pd metals, and n is the number of laminates in the stack, and is highly (100) textured through a non-epitaxial texturing method. The SP layer comprises at least one of an iron (Fe) layer, a cobalt (Co) layer, an alloy layer of cobalt iron (CoFe), a laminated layer of (Co/Fe)n, where n is the number of laminates in the stack.
The OB layer is formed by depositing an oxide layer, or by sequentially depositing a metal layer and oxidizing the metal layer. Alternatively, the oxide buffer layer may advantageously be formed by oxidizing the top surface of the pSAF immediately after the formation of the pSAF stack. In general, the OB layer has a thickness no more than 10 angstroms, preferably no more than 5 angstroms. The OB layer may strongly exchange couple the two magnetic moments that sandwich the OB layer so that their magnetization directions are parallel. The TS layer is formed by direct deposition on the OB layer and subsequent rapid thermal annealing (RTA) or fast quenching (H)). Alternatively, the TS layer is formed by direct deposition on the OB layer with an ultra-low deposition rate of no more than 0.5 angstrom per second and subsequent thermal annealing in hydrogen atmosphere. In general, the TS layer is highly (100) textured, and the SP layer grows epitaxially on the TS layer and is highly BCC (100) textured.
The pMTJ element further comprises a tunnel barrier directly on top of the cube-textured reference layer to permit spin-polarized current to pass between the tunnel barrier and the cube-textured reference layer, a magnetic recording structure directly on top of the tunnel barrier, and an oxide cap layer directly on top of the magnetic free layer, so that the element forms a pMTJ junction. The magnetic recording structure comprises a magnetic free layer, an oxide cap layer, a metal cap layer and a soft-magnetic adjacent layer. The free layer comprises at least one of an iron (Fe) layer, a cobalt (Co) layer, an alloy layer of iron cobalt (FeCo), a laminated layer of (Fe/Co)n, an alloy layer of FeX, a laminated layer of (Fe/X)n, where X represents Pt, Ni or Pd metals, and n is the number of laminates in the stack, and is highly (100) textured through a non-epitaxial texturing method. In one preferred embodiment, the free layer includes two BCC magnetic sub-layers that each includes material selected from the group consisting of ferromagnetic materials and ferrimagnetic materials; the free layer further includes one non-magnetic spacer layer between the two BCC magnetic sub-layers, in which the non-magnetic spacer layer exchange couples the two BCC magnetic sub-layers. The pMTJ element may contain no boron. The tunneling magnetoresistance of the junction may be at least 150%, or even at least 200% at room temperature. The soft-magnetic adjacent layer would reduce demag fields generated by domains of the magnetic free layer during its switching process, promote a coherent domain reversal of the magnetic free layer and correspondingly reduce its domain reversal time.
Alternatively, the TS layer comprises at least one selected from the group consisting of Fe/Y, FeCo/Y, (Fe/Co)n/Y, (Fe/Co)n/Fe/Y, (Fe/X)n/Y and (Fe/X)n/Fe/Y, where X represents Pt, Ni or Pd metals, Y represents W, Mo, Mg, Ta, or other Boron absorbing metals, and n is a positive integer, and is highly (100) textured through a non-epitaxial texturing method. The SP layer comprises an alloy layer of iron boron (FeB), an alloy layer of cobalt boron (CoB), an alloy layer of cobalt iron boron (CoFeB), or a combination thereof, where the composition of B is between 15% and 30%.
The perpendicular synthetic anti-ferromagnetic (pSAF) stack with a very strong PMA comprises a first magnetic pinning (FPL) layer having a FCC (111) textured crystalline structure and a second magnetic pinning (SPL) layer having a FCC (111) textured crystalline structure which sandwich a non-magnetic anti-ferromagnetic coupling (AFC) layer of Ru (or Rh, Ir) and are coupled into anti-parallel magnetization states through a very strong RKKY exchange interaction.
The application of said pMTJ element comprising above novel structures includes pSTT-MRAM as well as SOT-MRAM. Said pMTJ element in the invention is further sandwiched between an upper electrode and a lower electrode of each MRAM memory cell, which also comprises a write circuit which bi-directionally supplies a spin polarized current to the magnetoresistive element and a select transistor electrically connected between the magnetoresistive element and the write circuit.
The present application will now be described in greater detail by referring to the following discussion and drawings that accompany the present application. It is noted that the drawings of the present application are provided for illustrative purposes only and, as such, the drawings are not drawn to scale. It is also noted that like and corresponding elements are referred to by like reference numerals.
In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps and techniques, in order to provide an understanding of the various embodiments of the present application. However, it will be appreciated by one of ordinary skill in the art that the various embodiments of the present application may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the present application.
It will be understood that when an element as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “beneath” or “under” another element, it can be directly beneath or under the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly beneath” or “directly under” another element, there are no intervening elements present. Here, and thereafter throughout this application, each element written in the left side of “I” is stacked above an element written in the right side thereof.
In the invention, we propose a novel non-epitaxial growth method to fabricate a thin (100) textured starting (TS) layer over an oxide buffer (OB) layer, atop of a highly FCC (111) textured pSAF stack, which would later lead to epitaxial growth of a highly BCC (100) textured spin polarization layer, and to fabricate a (100) textured free layer.
In general, there is provided a pMTJ element comprising: a magnetically fixed pinning structure, a tunnel barrier layer, a magnetic free layer and a soft-magnetic adjacent layer. The pinning structure consists of a perpendicular synthetic anti-ferromagnetic (OAF) stack, an oxide buffer (OB) layer and a cube-textured reference layer. The pSAF stack includes a FCC (111) PMA material multilayer such as seed-layer/(Co/X)n/Co/Y/Co/(X/Co)n, where X represents Pt, Pd or Ni metals, m and ii are integers (normally m>n), and Y represents a Ru (or Rh, Ir) metal spacer. The cube-textured reference layer comprises a texture starting (TS) layer and a spin polarization (SP) layer. The TS layer includes a crystalline material selected from the group consisting of ferromagnetic materials and ferrimagnetic materials, while the SP layer, as deposited, includes a crystalline material or an amorphous material selected from the group consisting of ferromagnetic materials and ferrimagnetic materials. The TS layer comprises a highly BCC (100) textured material or a L10 superlattice (100) textured material after applying a non-epitaxial texturing method. The SP layer comprises a highly BCC (100) textured material. The thickness of the OB layer sandwiched between the pSAF stack and the cube-textured reference layer is thin enough to produce a strong magnetic parallel-coupling between the reference layer and the pSAF stack, so that the magnetization of the reference layer is pinned in a predetermined perpendicular direction. The TS layer comprises at least one selected from the group consisting of Fe, FeCo, Co, FeX, (Fe/Co)n, (Fe/X)n, Fe/Y, FeCo/Y, (Fe/Co)n/Y, (Fe/Co)n/Fe/Y, (Fe/X)n/Y and (Fe/X)n/Fe/Y, where X represents Pt or Pd metals, Y represents W, Mo, Mg, Ta, or other Boron absorbing metals, and n is a positive integer, and is highly (100) textured by applying a non-epitaxial texturing method. The SP layer includes at least one element selected from Fe, Co and B, and preferably comprises at least one selected from the group consisting of Fe, Co, CoFe, (Fe/Co)n, (Fe/CoFe)n, (Co/Fe)n, (Co/CoFe)n, (CoFe/Fe)n, (CoFe/Co)n, (Fe/Co)n/Fe, (Fe/Co)n/CoFe, (Fe/CoFe)n/Fe, (Fe/CoFe)n/Co, (Co/Fe)n/CoFe, (Co/Fe)n/Co, (Co/CoFe)n/Co, (Co/CoFe)n/Fe, (CoFe/Fe)n/Co, (CoFe/Fe)n/CoFe, (CoFe/Co)n/Fe, (CoFe/Co)n/CoFe, CoFe/(Fe/Co)n, Co/(Fe/CoFe)n, CoFe/(Co/Fe)n, Fe/(Co/CoFe)n, Co/(CoFe/Fe)n, Fe/(CoFe/Co)n, FeB, CoB, FeB/Fe, CoB/Fe, FeB/Co, CoB/Co, FeB/CoFe, CoB/CoFe, CoFeB, CoFeB/Fe, CoFeB/Co and CoFeB/CoFe, where n is a positive integer.
The OB layer is formed by depositing an oxide layer, or by sequentially depositing a metal layer and oxidizing the metal layer. Alternatively, the OB layer may advantageously be formed by oxidizing the top surface of the pSAF immediately in an oxidization chamber after the formation of the pSAF stack. In general, it is preferred that the OB layer is a magnetic oxide, such as FeO, CoO and FeCoO2, and has a thickness no more than 10 angstroms. The OB layer may strongly exchange couple the two magnetic moments that sandwich the OB layer so that their magnetization directions are parallel. The TS layer is formed by direct deposition on the oxide buffer layer and subsequent rapid thermal annealing (RTA) or fast quenching (FQ). The RTA or FQ can be conducted in-situ in a special chamber attached to a deposition system. As an iron rich layer is deposited directly over an oxide, Fe atoms get oxidized or partially oxidized on the surface, and a strong tensile stress is induced in the film plane, which is caused by interaction between Fe mono-layers and FeO. A sufficiently strong tensile strain in Fe film plane favors a BCC (100) texture. A rapid thermal annealing (RTA), in which lamps are used to heat a substrate quickly to high temperatures, induces an extra tensile strain in the film plane of the iron rich layer. The tensile strains from Fe/FeO interface and the RTA are added together, as a non-epitaxial texturing mechanism, and freeze the Fe film in the (100) texture. For a TS layer made of Fe or CoFe thin film deposited directly on an OB layer, a BCC (100) texture is promoted with the tensile strain in the film plane by performing the RTA within a proper heating/cooling rate range. For a TS layer made of (Fe/Pt)n thin film deposited directly on an OB layer, an L10 superlattice (100) texture is promoted with the tensile strain in the film plane by performing the RTA within a proper heating/cooling rate range. Alternatively, the TS layer is formed by direct deposition on the OB layer with an ultra-low deposition rate of no more than 0.5 angstrom per second and subsequent thermal annealing in hydrogen atmosphere. The thermal annealing in hydrogen atmosphere can be conducted in-situ in a special thermal annealing chamber attached to a deposition system. In general, the TS layer is highly (100) textured, and the SP layer grows epitaxially on the TS layer and is highly BCC (100) textured, the free layer is highly (100) textured.
Thin-film deposition methods offer the ability to control crystalline texture through epitaxial relationships with the substrate or a seed layer, and common deposition methods include molecular beam epitaxy (MBE), electron-beam (E-beam) evaporation, pulse-laser deposition (PLD) and plasma sputtering. Sputter deposition is the simplest method and widely used in labs and manufacturing industry, and normally sputtering rates are variable, ranging from slower than 1 Å/s to as fast as ˜1 μm/min.
The structure and morphology of a sputtered thin film can also change significantly during growth depending on substrate type and temperature, the available kinetic energy and the current state of the film. Theoretically, high adatom mobility and active surface diffusion allow a growing film to rearrange into energy minimizing configurations and achieve near-equilibrium structures. However, the ability to reconfigure is lost for fast deposition rates, and the film growth is said to be quenched if atomic mobility is too low or the deposition rate too high. Quenched growth modes or low substrate temperatures can kinetically inhibit energy-minimizing processes, yielding highly non-equilibrium as-deposited structures. Grain growth, coalescence, and defect elimination can induce in-plane tensile strain, relaxing any compressive strain and in some cases leading to a residual tensile strain state. Due to the non-equilibrium nature of magnetron sputtering it is often necessary to employ post-deposition processing to achieve a desired phase or microstructure. In another word, preferential grain growth occurs when there exists a larger driving force, such as additional in-plane strain, for the growth of one particular grain configuration of over any other. Such preferred configurations originate from large surface energy anisotropies or variable strain states of the growing crystallites. Besides, a close epitaxial relationship between the film and substrate or preceding layer can alter the strain in the growing film. Mismatched lattices lead to large strains as the film continues to grow. Both preferential and epitaxial growth can yield crystallographic texture. A rapid thermal anneal with high heating rates induces additional in-plane strain, activates diffusion, promotes grain growth and re-crystallization, and accelerates equilibration through enhanced kinetics. In fact, RTA is a class of techniques, including a spike annealing and a flash light annealing, that provide a way to rapidly anneal wafers to elevated temperatures for relatively short times, usually less than a minute. The heating is accomplished optically. An RTA system typically processes wafers singly and is capable of achieving process temperature of ˜200-1300° C. with a wide range of ramp rates ˜20-250° C. per second. Besides RTA, fast quenching processes include a laser quenching, an induction quenching, a flame quenching and a carburizing quenching. Among them, the laser quenching has a high power density and fast heating/cooling rates, which would also serve the same purpose as the RTA. In another word, the TS layer can also be formed by direct deposition on the oxide buffer layer and subsequent laser quenching or other fast quenching (FQ) process.
While metal films and adjacent oxide films are thin, epitaxial relationship is generally maintained between them. The perfect epitaxy cannot, however, be expected because of the lattice mismatch between them, thus the both lattices are strained. For an iron thin film growing over a ferrous oxide (FeO) thin film, a tensile strain is induced in the Fe film, although the FeO lattice contracts to some extent, i.e., the FeO is in compression and correspondingly the Fe film is in tension. Fe films completely cover oxide substrates for thicknesses of a few ML, indicating that Fe “wets” the surface better than other elements such as Cu. The ability of the metal film to cover (wet) the oxide substrate correlates directly with the reactivity of the metal toward oxygen. For Fe films, there is a strong interaction between the films and the oxide substrate, i.e., oxidation of the metal over-layers and reduction of the substrate, at sub-monolayer thickness. Moreover, a rapid thermal annealing (RTA), or a laser quenching, induces a strong tensile strain in Fe thin films on oxide substrate, and promotes (100) ordering in Fe films. In another word, after a RTA or a laser quenching, a thin Fe film on an oxide substrate freezes in a (100) preferred orientation. Similarly, this is also true for CoFe to obtain BCC (100) orientation, and for FePt to obtain L10 ordering as well as (100) orientation.
In next sections, three embodiments of manufacturing pMTJ elements according to this invention are present.
After forming the pSAF stack (1234), an OB layer (15) is deposited on top of the pSAF, as shown in
After the RTA, the TS layer serves as a highly (100) texture seed layer for the SP layer to achieve a BCC (100) texture. The SP layer is one selected from the group consisting of CoFe, CoFe/Fe, (Fe/Pt)n/CoFe, (Fe/Pd)n/CoFe, (Fe/Pt)n/CoFe/Fe and (Fe/Pd)n/CoFe/Fe.
After forming the pSAF stack (1234), a metal layer (15M) is deposited on top of the pSAF, as shown in
After the FQ process, the TS layer serves as a highly (100) texture seed layer for the SP layer to achieve a BCC (100) texture. The TS layer is one selected from the group consisting of CoFe, CoFe/Fe, (Fe/Pt)n/CoFe, (Fe/Pd)n/CoFe, (Fe/Pt)n/CoFe/Fe, and (Fe/Pd)n/CoFe/Fe.
In the third embodiment, the pSAF stack is directly oxidized after the pSAF stack is formed.
After the RTA, the TS layer serves as a highly (100) texture seed layer for the SP layer to achieve a BCC (100) texture. The SP layer is one selected from the group consisting of CoFe, CoFe/Fe, (Fe/Pt)n/CoFe, (Fe/Pd)n/CoFe, (Fe/Pt)n/CoFe/Fe, and (Fe/Pd)n/CoFe/Fe.
As an option, The TS layer consists of at least a first sub-layers and a second sub-layer, wherein the first sub-layer is preferred to be one selected from the group consisting of Fe, FeCo, Co, [Fe/Co]n and [Fe/Pt]n, and the second sub-layer contains Boron absorbing material, such as W and Mo. In this case, the SP layer can be one selected from the group consisting of CoFe, CoFeB, CoFe/Fe, CoFeB/Fe, (Fe/Pt)n/CoFe, (Fe/Pd)n/CoFe, (Fe/Pt)n/CoFe/Fe, and (Fe/Pd)n/CoFe/Fe.
The non-epitaxial texturing method described as above can be generalized to a tensile strain quenching (TSQ) technique which also comprises other quenching methods such as a pressure quenching (PQ) process. The TSQ technique can rapidly produce a tensile stress/strain in the TS film plane at a high temperature and correspondingly induce an energetically favored (100) texture, and have a successful retention of the (100) texture in the final TS layer as the temperature rapidly decreases to a room temperature. Other embodiments of this invention may include using any quenching technique, even at various processing stages. For example, the cube-textured reference layer may have the SP layer combined into the TS layer, and the TS layer is sandwiched between the OB layer and the tunnel barrier layer. The rapid thermal annealing, or the laser quenching, or any other TSQ techniques, is preferred to be applied after the formation of the tunnel barrier layer. Because the TS layer is sandwiched between two oxide layers, the quenching induced tensile strain is much larger than the one that a quenching process is applied immediately after the deposition of the TS layer. Similarly, the TSQ technique may also apply to formation of free layers in pMTJs. For example, if a free layer is boron-free and comprises at least one of an iron (Fe) layer, a cobalt (Co) layer, an alloy layer of cobalt iron (CoFe), an alloy layer of cobalt nickel iron (CoNiFe), an alloy layer of cobalt nickel (CoNi), an alloy layer of iron platinum (FePt), an alloy layer of iron palladium (FePd), an alloy layer of iron nickel (FeNi), a laminated layer of (Fe/Co)n, a laminated layer of (Fe/CoFe)n, a laminated layer of (Fe/Pt)n, a laminated layer of (Fe/Pd)n and a laminated layer of (Fe/Ni)n, a TSQ process may be applied in-situ in a special chamber attached to a deposition system after formation of an oxide cap layer. If a free layer comprises a first free sub-layer which is boron-free and a second free sub-layer which contains boron, a TSQ process may be applied immediately after formation of the first free sub-layer. A boron-free magnetic free layer has a strong exchange stiffness constant, which is expected to have a more uniform magnetic domain and a more rapid magnetic domain switching speed when a spin-polarized write current is applied during a write process. For example, the exchange stiffness constant of CoFeB with 20% B composition is only about between 30% and 50% of the exchange stiffness constant of pure CoFe.
The pMTJ element further comprises a soft-magnetic adjacent layer directly on top of the pMTJ junction, as shown in
A perpendicular magnetic tunnel junction (pMTJ) element as described above may also be used in a spin-orbit torque magnetoresistive random-access memory (SOT-MRAM). In one embodiment, it comprises: a seed layer; a high-resistive soft-magnetic adjacent layer atop the seed layer; a spin-orbit torque (SOT) layer atop the soft-magnetic adjacent layer; an oxide buffer layer atop the SOT layer; a magnetic free layer atop the oxide buffer layer, the magnetic free layer having a free layer magnetization that is perpendicular to the magnetic free layer and is changeable; a tunnel barrier layer atop the magnetic free layer; and a magnetic reference layer atop the tunnel barrier layer, the magnetic reference layer having a reference layer magnetization fixed in a direction perpendicular to the magnetic reference layer, wherein the magnetic free layer comprises a crystalline material selected from the group consisting of ferromagnetic materials and ferrimagnetic materials and contains no boron element, the oxide buffer layer comprises a metal oxide, the soft-magnetic adjacent layer comprises at least one high-permeability material layer having a near-zero magnetostriction, and the vertical distance between the magnetic free layer and the soft-magnetic adjacent layer is no more than 5.0 nm. It may further comprise an oxide insertion layer between the SOT layer and the soft-magnetic adjacent layer. The TSQ technique may also apply to formation of free layers in a SOT-MRAM. For example, if a free layer is boron-free and comprises at least one of an iron (Fe) layer, a cobalt (Co) layer, an alloy layer of cobalt iron (CoFe), an alloy layer of iron platinum (FePt), an alloy layer of iron palladium (FePd), an alloy layer of iron nickel (FeNi), a laminated layer of (Fe/Co)n, a laminated layer of (Fe/CoFe)n, a laminated layer of (Fe/Pt)n, a laminated layer of (Fe/Pd)n and a laminated layer of (Fe/Ni)n, a TSQ process may be applied in-situ in a special chamber attached to a deposition system immediately after formation of the tunnel barrier layer.
While certain embodiments have been described above, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
This application is a continuation in part of the U.S. utility patent application Ser. No. 17/373,757 entitled PERPENDICULAR MTJ ELEMENT HAVING A CUBE-TEXTURED REFERENCE LAYER AND METHODS OF MAKING THE SAME, filed on Jul. 12, 2021, and the entire contents of which are incorporated herein by reference.
